Embodiments of the present invention relate to a detector comprising first and second lenses for use with respective first and second sensing means; each lens comprising a plurality of Fresnel facets having respective fields of view adapted such that the fields of view of the first lens are alternately arranged with the fields of view of the second lens such that the fields of view of the first lens are adjacent only to, but do not overlap with, the fields of view of the second lense in a single direction.